The present invention relates to the common venting of separate apparatus having exhaust gases discharged at different vent pressures.
It is accepted practice to vent several natural-draft combustion apparatus through a common vent or chimney. For example, draft hood equipped furnaces and water heaters are typically vented through a common chimney in residential applications. However, a positive vent pressure apparatus is separately vented due to the risk of flue gas spillage through the draft hood of other apparatus being vented. Thus, manufacturers of residential high-efficiency furnaces with forced-draft or fan-assisted burner systems recommend the use of a separate vent while most water heaters presently marketed use a draft hood. (The term furnace is used in a broad sense herein to include a boiler or other heating arrangement unless prohibited by the context of use. The resulting venting system is more complex, expensive and difficult to install.
When an existing natural-draft furnace is replaced with a separately vented forced-draft furnace, the chimney through which the original furnace and water heater were common vented may be oversized for the water heater alone. ln such cases, the elimination of the flow of furnace draft air and exhaust gases through the chimney may result in excessive cooling and condensation of the water heater exhaust gases being vented. The chimney may not be designed to accommodate the additional condensate and venting of the gases may be impeded by the reduced buoyancy of the additionally cooled exhaust gases.
The use of fan-assisted or natural and forced-draft venting systems in a single apparatus are disclosed in U.S. Pat. Nos. 4,487,137 and 2,497,944. These systems may also include on-off fan controls responsive to flue gas temperature and adjustable damper arrangements.
The use of fans or blowers to provide venturi or jet pump flows within single apparatus vent stacks to enhance gas flow or dilute exhaust gases is disclosed in U.S. Pat. Nos. 3,585,946, 3,570,423, 3,448,917, 1,604,271 and 1,533,898. U.S. Pat. No. 2,397,870 teaches the use of a fan to provide a venturi-aspirator effect to induce atmospheric air flow into a single apparatus vent stack in order to reduce the flue gas temperature.
U.S. Pat. No. 4,149,453 discloses the addition of diluent gas with and without heating to exhaust gases in order to reduce condensation by maintaining the exhaust gases from a single apparatus above their dew point.
U.S. Pat. No. 1,869,939 discloses reducing the relative humidity and condensation of exhaust gases from a single apparatus in a fan-powered burner by injection of a flow of air from the burner fan.